This invention relates to shale based gas and oil recovery methods and fracture water treatment.
The gas industry has developed techniques to recover natural gas from shale deposits by so called “horizontal fracturing.” In horizontal fracturing, a stream of water is injected under pressure into wells disposed through shale deposits. The fracturing process uses, e.g., from 1 to 3 million gallons of water per fracture effort. Once the fracturing is completed, the water (i.e., “frac water”) is contaminated with petroleum residue and is returned to holding tanks for decontamination. In the holding tanks, the return water settles layers comprising a clear petroleum product, e.g., light non-aqueous phase liquids (LNAPL), which are liquids that are nearly insoluble in water and less dense than water and an underlying water product. For example, oil, gasoline, etc. are examples of LNAPLs. This LNAPL, clear petroleum product, is normally decanted off into tanks or a lined reservoir and sold to refineries as a petroleum product.
The underlying water layer is contaminated, e.g., by high concentrations of alkanes, and may be somewhat saline. Generally, the underlying water contains from 1 to 4 parts per thousand chlorides (10,000-40,000 ppm) after usage. Other ranges are of course possible based on environmental conditions. Normally, drinking water standards generally require levels less than 250 ppm chloride, in order to have proper viscosity for discharge, whereas river discharge standards require generally levels less than 2,000 ppm, although this standard can vary.